1. Field of the Invention
The present invention relates to an electromagnetic clutch which is preferably applicable to a compressor in a vehicle air conditioner.
2. Related Art
Conventionally, a compressor in a vehicle air conditioner is driven by power transmitted from a vehicle engine through an electromagnetic clutch, and operation of the compressor is controlled by turning on and off the clutch. However, when the compressor seizes due to some reason, a movable portion of the compressor may seize and stick, thereby locking the compressor. In this case, it is required to turn off the clutch so that power transmission from the engine to the compressor is stopped to protect the engine.
JP-A-57-51025 discloses an electromagnetic clutch used for a compressor and having a thermal fuse connected in series to an electric circuit therein. When the compressor is locked, the thermal fuse is fused by frictional heat generated between an armature and a rotor housing of the clutch so that the clutch is turned off.
However, in the above-mentioned clutch, the thermal fuse needs to be directly connected to the electric circuit adjacent to an exciting coil in the clutch. Therefore, the number of manufacturing processes of the clutch is increased, thereby decreasing productivity of the clutch and increasing production cost of the clutch. Further, the clutch needs not only a space for accommodating the thermal fuse but also a space for accommodating a connection portion between the thermal fuse and the electric circuit. As the result, a size of the clutch is increased.
The number of windings of the exciting coil may be decreased so that the size of the clutch is reduced. However, in this case, magnetomotive force induced by the exciting coil is decreased. As a result, electric power supplied to the exciting coil may need to be increased so that magnetomotive force of the exciting coil is restricted from decreasing. However, when power supply to the exciting coil is increased, power consumption of the exciting coil is also increased.